An elevator comprises an elevator car and a counterweight attached to each other by a series of ropes. The ropes extend up the hoistway from the elevator car to the machine room of the elevator. In the machine room, the ropes wrap around a sheave attached to an elevator drive and return down the hoistway attaching to the counterweight. In an elevator with a geared elevator drive, a drive motor drives the sheave through a intermediary gear arrangement. In an elevator with a gearless drive, conversely, the sheave is fixed to the rotational axis of the drive motor. Consequently, the drive motor directly drives the sheave, hence the "gearless" drive.
For safety reasons, elevators are generally required to have an overspeed governor and safeties. Early governor embodiments included a governor rope extending the length of the hoistway, attached to a governor sheave and tensioner. If the downward velocity of the elevator exceeded a predetermined limit, an overspeed condition, a centrifugal flyweight assembly driven by the governor sheave would swing outwardly, tripping a switch thereby removing power to the elevator drive and brake. If the downward elevator speed continued to increase, the flyweight assembly would swing outwardly still further and operate a governor brake. The governor brake would apply a frictional drag force to the governor rope, thereby actuating a pair of coordinated safeties in communication with the rope. The safeties, attached to the elevator car, acted on a pair of rails guiding the elevator. This entirely mechanical system, while effective, presented a number of problems.
First, the governor assembly and safeties protected against overspeed conditions only when the elevator car was descending. In the event of a brake failure or a drive gear failure in a geared machine, for example, a heavier counterweight will cause a lighter elevator car to accelerate upwardly. The unidirectional limitation of the aforementioned governor and safeties renders them powerless to stop an upwardly accelerating car.
Second, the centrifugal nature of the governor makes the governor inoperable at low elevator speeds. If an elevator leaves a landing with an open door because of a faulty brake, for example, a centrifugally operated governor will not stop the elevator car until it has reached an overspeed condition.
Third, the complete governor assembly was costly and burdensome to maintain due in part to the high-wear nature of some of the assembly's elements. For example, the governor brake typically comprised a swinging jaw mechanism that pinched the governor rope. As a result, the governor rope was subject to undesirable wear. In addition, the safeties actuated by the governor rope stopped the car by scoring the rail.
U.S. Pat. No. 4,977,982 discloses an "Elevator Sheave Brake Safety" comprising an electromechanically actuated overspeed governor for use with either geared or gearless drive elevators. Unlike the aforementioned early embodiments, this patent employs a pair of wedge brakes operating against a brake surface attached to the drive. The wedges are maintained in the "off" mode by an energized solenoid which receives its power based on a signal from a peripherally mounted speed detecting means. In the event of an overspeed condition, the solenoid is de-energized and the wedges are biased against the brake surface. Depending on the rotational direction of the brake surface, one of the brake wedges is drawn into engagement with the brake surface, thereby stopping the drive. The other wedge is maintained out of engagement with the brake surface. Rotation of the brake surface in the opposite direction results in the engaging wedge brake and the non-engaging wedge brake trading places.
Hence, one of the advantages of U.S. Pat. No. 4,977,982 is its capability to control overspeed conditions in either direction. Another advantage of U.S. Pat. No. 4,977,982 is that it eliminates the need for almost all of the governor hardware including the governor sheave, rope and tensioner. The elimination of these conventional devices saves money on both the initial installation and continued maintenance of the elevator. Moreover, it also saves a considerable amount of room in the always congested hoistway.
Changing code requirements and preferences, however, may favor a mechanically operated governor with the aforesaid advantages, rather than the above described brake which utilizes electrical solenoids and speed detecting means.